The present invention finds a particularly advantageous but in no way limiting application in ultra-narrowband wireless communication systems. “Ultra-narrowband” (or UNB in the literature) is understood to mean that the instantaneous frequency spectrum of the radio signals transmitted by the terminals has a frequency width of less than one kilohertz.
Such UNB wireless communication systems are particularly suitable for M2M (“Machine-to-Machine”) or Internet of Things (or IoT in the literature) applications.
In such a UNB wireless communication system, data exchanges are essentially unidirectional, specifically on an uplink between terminals and an access network of said system.
The terminals transmit uplink messages that are collected by base stations of the access network, without having to associate themselves with one or more base stations of the access network beforehand. In other words, the uplink messages transmitted by a terminal are not intended for a specific base station of the access network, and the terminal transmits its uplink messages assuming that they will be able to be received by at least one base station. Such provisions are advantageous in that the terminal does not need to carry out regular measurements, which are intensive in particular from a power consumption point of view, in order to determine the most appropriate base station for receiving its uplink messages. The complexity hinges on the access network, which must be capable of receiving uplink messages that are able to be transmitted at arbitrary points in time, and on arbitrary center frequencies within a multiplexing frequency band of the various terminals.
Such a mode of operation, in which the data exchanges are essentially unidirectional, is entirely satisfactory for numerous applications, such as for example the remote reading of gas, water and electricity meters, the remote monitoring of buildings or houses, etc.
In some applications, however, it may be advantageous also to be able to perform data exchanges in the other direction, namely on a downlink from the access network to the terminals, for example in order to reconfigure a terminal and/or control an actuator linked to said terminal. However, it is necessary to offer such a capability while limiting the complexity of the terminals.
U.S. Pat. No. 6,130,914 describes an example of a bidirectional UNB wireless communication system making it possible to limit the complexity of the terminals. Specifically, in U.S. Pat. No. 6,130,914, the center frequency on which the access network transmits a downlink message to a terminal, in response to an uplink message transmitted by said terminal, is determined by said access network on the basis of the center frequency on which said uplink message has been received.
The precision in the generation of the center frequency of the uplink message, on the terminal side, may thus be poor and achieved using inexpensive frequency synthesizing means. Specifically, the access network does not know a priori on which center frequency the uplink message is transmitted, and by default has to listen to the uplink over the entire multiplexing frequency band in order to detect this uplink message. The center frequency of the detected uplink message is estimated by the access network, and the terminal and the access network then have substantially the same reference frequency, specifically the center frequency of the uplink message, to generate the center frequency of the downlink message. In other words, the terminal and the access network are then substantially frequency-synchronized, this being the case regardless of the precision of the frequency synthesizing means of the terminal, since it is the access network that frequency-synchronizes itself with said terminal, and not the other way round.
Furthermore, since the terminal knows, on the basis of the center frequency of the uplink message that it transmitted itself, how to determine the center frequency on which the downlink message will be transmitted, it is able to listen to the downlink solely around said predetermined center frequency of the downlink message, over a frequency band with a width of the order of the instantaneous spectral width of the downlink message, which width is much smaller than the width of the multiplexing frequency band.
In contrast to the access network, the terminal therefore does not have to listen to the entire multiplexing frequency band. What is more, given that the downlink message is transmitted in response to an uplink message transmitted by the terminal, the latter does not have to listen to the downlink permanently, but only after having transmitted an uplink message.
A problem arises, however, when interference is present in the multiplexing frequency band.
Specifically, if the terminal selects the center frequency of the uplink message without checking the availability of said frequency, this being advantageous for relieving said terminal from having to carry out regular measurements on the uplink, then the uplink message may be missed by the access network. Furthermore, the access network does not transmit a downlink message and the terminal nevertheless listens to the downlink, awaiting a downlink message.